Protic Ionic Liquids as p-Dopant for Organic Hole Transporting

Subscriber access provided by Lulea University of Technology

Article

Protic Ionic Liquids as p-dopant for Organic Hole Transporting Materials and their Application in high efficiency Hybrid Solar Cells Antonio Abate, Derek J. Hollman, Joël Teuscher, Sandeep Pathak, Roberto Avolio, Gerardino D'Errico, Giuseppe Vitiello, Simona Fantacci, and Henry J. Snaith J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja406230f • Publication Date (Web): 25 Jul 2013 Downloaded from http://pubs.acs.org on July 26, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Journal of the American Chemical Society is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Protic Ionic Liquids as p-dopant for Organic Hole Transporting Materials and their Application in high efficiency Hybrid Solar Cells Antonio Abate1, Derek J. Hollman1, Joël Teuscher2,†, Sandeep Pathak1, Roberto Avolio3, Gerardino D’Errico3, Giuseppe Vitiello,4 Simona Fantacci5, Henry J. Snaith1*

1

Clarendon Laboratory, Department of Physics, University of Oxford, Parks road, Oxford, OX1 3PU, United Kingdom 2

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École polytechnique fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland 3

Institute of Polymer Chemistry and Technology (ICTP), National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy 4

University of Naples "Federico II", Department of Chemical sciences, Via Cinthia, 80126 Napoli, Italy

5 Istituto CNR di Scienze e Tecnologie Molecolari (CNR-ISTM), c/o Dipartimento di Chimica, Universia’ di Perugia, Via elce di Sotto 8, I-06213 Perugia, Italy

†

Present address: Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École polytechnique fédérale de Lausanne (EPFL), Station 6, CH-1015 Lausanne, Switzerland

* [email protected]

KEYWORDS: Protic Ionic Liquids, p-doping, Organic Semiconductors, Hybrid Solar Cells, Dye-Sensitized Solar Cells, Perovskite.

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

ABSTRACT Chemical doping is a powerful method to improve the charge transport and to control the conductivity in organic semiconductors (OSs) for wide range of electronic devices.

We

demonstrate protic ionic liquids (PILs) as effective p-dopant in both polymeric and small molecule OSs. In particular, we show that PILs promote single electron oxidation, which increases the hole concentration in the semiconducting film.

The illustrated PIL-doping

mechanism is compatible with materials processed by solution and is stable in air. We report the use of PIL-doping in hybrid solar cells based on triarylamine hole transporting materials such as Spiro-OMeTAD.

We show improved power conversion efficiency by

replacing lithium salts, typical p-dopants for Spiro-OMeTAD, with PILs. We use photovoltage photocurrent decay and photoinduced absorption spectroscopy to establish that significantly improved device performance is mainly due to reduced charge transport resistance in the holetransporting layer, as potentiated by PIL-doping.


Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Organic semiconductors (OSs) are entering the electronics market in various stages of development for a range of applications and are expected to grow significantly in the near future. Key factors driving penetration of organic electronics in the mainstream market are their low cost of fabrication and their high degree of flexibility.1 Since their discovery in 1977,2 several studies have shown the possibility of tuning their properties for specific applications by chemical doping.3 Chemical doping may be achieved by introducing “impurities” into the OSs, which increases the density of mobile charge carriers and thus conductivity; through this, doped organic semiconductors operate conceptually similar to traditional inorganic semiconductors.

In

practice, chemical doping can involve several mechanisms depending on both the semiconductor and the dopant.3

Doping of polyacetylene via protonation was a principal system of

semiconductor chemical doping understood by immersion of polyacetylene in aqueous HCl.4 Upon drying the polymer, residual H3O+ groups remain within the organic matrix, increasing the density of mobile charges by inductively withdrawing electrons from the electron-rich conjugated polymer chains; the resulting p-doped OS showed an increase in conductivity of ten orders of magnitude.4 Following this study, aqueous H+ treatment of OSs has been reported as a general doping mechanism for many conjugated polymers.5-9 Recently, several heterocyclic -

organic hydrides10 have been reported as efficient n-type dopants by releasing hydride (H ) or atomic hydrogen (H●).11 In particular, 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole derivatives (DMBI) can increase the conductivity of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by more than four orders of magnitude through this chemical doping treatment.11 Following H● release, highly energetic DMBI radicals reduce PCBM, increasing the density of mobile charge carriers in the OS matrix. The stability of this doping mechanism depends on the rate of the back electron transfer from the doped PCBM to the DMBI cation, which can be stabilized by an electron-rich substituent on DMBI core.12 This mechanism has been effectively applied to prepare air-stable organic thin film transistors.12 Many other organic electronic applications can benefit from chemical doping; for example triphenylamine-based OSs have been p-doped with Co(III) complexes in solar cells13 and tetracyano-quinoline derivatives in organic light emitting diodes.14

Both systems benefit from the introduction of a p-doped

transport layer, reducing the charge transport series resistance with the photoactive junction13,1517

and helping to forge Ohmic contacts at the electrode interfaces.18-20 Furthermore, chemical

doping may also facilitate the fabrication of tandem structures for efficient organic solar cells using a versatile recombination contact.21 However, most of the dopants reported in scientific



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

literature have never been exploited for electronic applications since their application was incompatible with cost-effective processing or their doping effects were weak and reversible during the device operation. In this work, we report the use of protic ionic liquids (PILs)22 as a protonation generator to describe a new, efficient, and air-stable p-doping mechanism. We show that PILs, used as a source of H+, are effective dopants in small molecule OSs.5-8,23 We report both experimental and theoretical analyses to establish a PIL-doping mechanism in OSs. In particular, we show that bis(trifluoromethanesulfonyl)imide (H-TFSI), as source of H+, increases the conductivity of 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9’-Spirobifluorene

(Spiro-OMeTAD)

by

three orders of magnitude. Additionally, we take advantage of PIL-doping to demonstrate solidstate dye-sensitized solar cells (ss-DSSCs)24 and meso-superstructured solar cells (MSSCs)25,26 with improved photon to electron power conversion efficiency compared to the state-of-the-art. We use photovoltage - photocurrent decay and photoinduced absorption spectroscopy to establish that significantly improved device performance is mainly due to reduced charge transport resistance in the hole-transporting matrix, as potentiated by PIL-doping.


Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RESULTS AND DISCUSSION Recently, numerous studies have focused on ionic liquids both as solvents and as electrolytes for electronic applications.27 This interest is due to their remarkable properties such as negligible vapour pressures, low melting points, non-flammability, and a significant ability to solvate many organic and inorganic chemicals.28 In the broad range of ionic liquids, protic ionic liquids (PILs) are Brønsted acid-base combinations, wherein the proton transfer from acid to base creates the anion-cation couple.22 The archetypal PIL is bis(trifluoromethane)sulfonimide (H-TFSI, see Figure 1a), in which the cation is H+ and the anion is CF3SO2-N--SO2CF3. In this particular anion, the extremely electron-withdrawing CF3SO2- groups reduce the basic character of the imide nitrogen (-N-), distributing the negative charge over multiple atoms.29

As

consequence of this, the self-dissociation of H-TFSI produces “free” protons in organic solution, likewise in aqueous HCl solution.30 Thus, organic molecules dissolved in H-TFSI solutions have a high probability of being protonated and stable organic cations may be generated, coupled with a TFSI anion.22 The rate of proton transfer can be controlled by tuning the acidic character of PIL.30 Indeed, in addition to H-TFSI, several N-heterocyclic ionic liquids may be used as weak Brønsted acid.31 In particular, 1-alkyl-3-methylimidazolium bis(trifluoromethane)sulfonimide (Himi-TFSI, see Figure 1a) has been shown to produce stable carbenes by means of proton release from the aromatic ring.32 Nevertheless, the self-dissociation constant for Himi-TFSI is much lower than H-TFSI, as symbolically indicated on the pH scale in Figure 1a.30 Also in Figure 1a, we reported an aprotic ionic liquid, tetraethyl bis(trifluoromethane)sulfonimide (Et4NTFSI), which can be assigned as a weak Brønsted base.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) Chemical structure of ionic liquids used in this study, bis(trifluoromethane)sulfonimide (H-TFSI) and 1-alkyl-3-methylimidazolium bis(trifluoromethane)sulfonimide (Himi-TFSI, n = 6 for the data reported in this work) are protic ionic liquids (PILs), with H-TFSI more acidic (free proton in red) than Himi-TFSI (the most acidic proton in orange),22 and tetraethyl bis(trifluoromethane)sulfonimide (Et4N-TFSI) as an aprotic ionic liquid; the pH scale may be considered as an indication of how strongly a proton will be transferred from the PIL to a base, though it must be noted that the pH is usually considered for aqueous solutions and may not be appropriate for the non-aqueous PILs. (b) Chemical structure of the small molecule organic semiconductor used in this study 2,2’,7,7’tetrakis(N,N-di-p-methoxyphenyl-amine)9,9’-Spirobifluorene (Spiro-OMeTAD). (c) Conductivity against the content of different ionic liquids, the dashed lines indicate the conductivity of the undoped and the Co(III) doped13 Spiro-OMeTAD as measured in our laboratory. (d) UV-Vis absorption spectra of Spiro-OMeTAD doped with H-TFSI and the absorption intensity at specific wavelengths as function of the H-TFSI concentration (inset). The peaks at 502 and 690 nm have been assigned to oxidized Spiro-OMeTAD (Spiro-OMeTAD+ or Spiro-OMeTAD2+).33 To demonstrate the p-doping effect of PILs, we focus our experiments on the well-known small molecule OS, Spiro-OMeTAD (see Figure 1b), a state-of-the-art organic hole transporting material in hybrid solar cells.13,25 However, recent studies have demonstrated that the slow


Page 6 of 32

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


charge transport and the ensuing low conductivity of Spiro-OMeTAD significantly limits the power conversion efficiency (PCE) of the device.34

Indeed, Burschka and co-workers

significantly enhanced the PCE in ss-DSSCs by using Co(III) complexes as chemical dopant, increasing the conductivity of Spiro-OMeTAD by two orders of magnitude.13 PIL-doping mechanism. In Figure 1c we reported the conductivity of Spiro-OMeTAD as function of the ionic liquid content, as measured in inert atmosphere (nitrogen) and dark condition.

A fast growth is immediately visible for both H-TFSI and Himi-TFSI at low

concentration, which reaches a plateau around 10 mol % with Spiro-OMeTAD. Notably, HTFSI can enhance the conductivity by up to three orders of magnitude compared to the undoped OS.

Et4N-TFSI shows that the addition of aprotic ionic liquids in the organic matrix has a

detrimental effect on the conductivity. We should nevertheless note the increase in conductivity at very high concentration of Et4N-TFSI (30 mol % with Spiro-OMeTAD). This effect, which can occur regardless the presence of “free” proton, could be attributed to the electrostatic charge of the ionic species added into the organic matrix, as discussed in previous reports.35 However, to prove the diverse nature of doping with protic and aprotic ionic liquids, in Figure 1d we reported the UV-Vis absorption spectra for the Spiro-OMeTAD films doped with H-TFSI. There is a clear growth of absorption bands at 502 nm and at 690 nm and a simultaneous decrease of the bands at 294 nm and at 370 nm, as the concentration of H-TFSI is increased. As already observed for the conductivity, the absorption changes very rapidly at low H-TFSI content, reaching a plateau around 10 mol %. A clear trend for Himi-TFSI doped films could not be obtained,

as the absorption at 510 nm and 690 nm was too weak (see Supporting

Information).. The same spectra for Et4N-TFSI showed only the bands at 294 nm and at 370 nm without any indication of absorption at both 510 and 690 nm, even with high dopant concentration. These absorption modes in the Spiro-OMeTAD have already been assigned via experimental and computational studies to the neutral molecule (Spiro-OMeTAD, 294 and 370 nm) and first or second oxidized state (Spiro-OMeTAD+ or Spiro-OMeTAD2+, 502 and 690 nm).24,,34

Thus, we have evidence that PILs can oxidize (p-dope) Spiro-OMeTAD

depending on their ability to release protons. We also note a weak absorption band at 575 nm, which has not been previously observed in oxidized Spiro-OMeTAD33,34, and it may therefore belong to new species generated by PIL doping, which we will investigate below.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) 1H NMR collected in deuterated dimethyl sulfoxide for Himi-TFSI and SpiroOMeTAD doped with Himi-TFSI show a strong downfield shift of the peak belonging to the most acidic hydrogen atom (highlight in Figure 1a) of the imidazolium cation from 8.7 to 9.7 ppm, which is indicative of interaction with a Lewis base. (b) TDDFT computed absorption spectra of the Spiro-OMeTAD protonated on nitrogen (Spiro-OMeTAD-NH+, black dashed line), the reduced species of the protonated Spiro-OMeTAD (Spiro-OMeTAD-NH● red line) and of the oxidized Spiro-OMeTAD (Spiro-OMeTAD+ black solid line). The DFT optimized geometry of Spiro-OMeTAD-NH; the added proton is colored purple and is encircled with red. The spectral absorption band of the Spiro-OMeTAD-NH species has been highlighted blue and the TDDFT excitations reported. (c) EPR spectrum and influence of microwave power on spectrum amplitude of H-TFSI doped Spiro-OMeTAD powder. (d) Proposed scheme of PIL doping mechanism. To shed light on the PIL doping mechanism, we first demonstrate the protonation of SpiroOMeTAD through measuring 1H NMRspectra (Figure 2a).

While the spectrum of Spiro-

OMeTAD in deuterated dimethyl sulfoxide gives sharp peaks (see Supporting Information), upon addition of PILs to the solution, most of the peaks become deformed, substantially broadened, or completely quenched. This may be due to paramagnetic radical species generated after the addition of PIL, such as the formation of Spiro-OMeTAD+ that we described in Figure 1d. Particularly, when we added a strong acidic PIL such as H-TFSI, no signals from SpiroOMeTAD could be measured (the TFSI ion do not contain hydrogen) while a weaker acidic PIL


Page 8 of 32

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


such as Himi-TFSI still allowed the identification of the collapsed peaks at 3.7, 6.8, and 6.9 ppm along with the spectrum of Himi+(see Experimental Section for more details). The persistence of PIL 1H resonances suggests that the quenching of the signals for Spiro-OMeTAD is not solely due to the fast-relaxation effect of paramagnetic species in solution.36 Indeed, in this case we should see a similar deformation for the resonances of the entire spectrum rather than specifically for Spiro-OMeTAD.36 As previously reported,37-39 such specific quenching may be ascribed to one-electron oxidation of the protonated molecule, which generates paramagnetic radical species in addition to Spiro-OMeTAD+.40 However, radicals species may also strongly deform the 1H signals of the remaining molecules in solution, depending on their relative concentration.37 Subsequently, in order to reduce the concentration of the radical species, we focus our 1H NMR experiments on Himi-TFSI, which is a significantly weaker acid than H-TFSI (see Figure 1). In Figure 2a we report the spectra for Himi-TFSI in the presence and absence of Spiro-OMeTAD to demonstrate the proton exchange reaction. The peak of the most acidic hydrogen atom, highlighted in Figure 1a, showed a strong downfield shift, from 8.75 ppm to 9.66 ppm upon addition of Spiro-OMeTAD.41 Notably, the remaining peaks from both HimiTFSI and Spiro-OMeTAD remained unaffected, as detailed in the Experimental Section. Such behaviour is compatible with a partial or a complete proton transfer from the imidazolium cation to a basic site of the Spiro-OMeTAD.41 The possible basic centers of the Spiro-OMeTAD are the triphenylamine nitrogens and the etheric oxygens. We performed density functional theory (DFT) calculations to extract the stability of the structures with one proton alternatively bound to nitrogen or to oxygen centers. The species protonated a nitrogen atom (Figure 2b), hereafter labeled as Spiro-OMeTAD-NH+, is markedly stabler than the species protonated on an oxygen atom by as much as 20.35 kcal/mol. The optimized structures of both the protonated species and the relative formation energies are reported in the Supporting Information. As hypothesized, protonation occurs on the most basic center of the system, although the nitrogen atoms are quite sterically congested while the oxygen sites are more accessible. Time-dependent DFT (TDDFT) calculations were previously shown to accurately simulate the absorption spectra of the neutral Spiro-OMeTAD and its oxidized species.33 Here, we computed the spectra of Spiro-OMeTAD-NH+ and the possible related reduced species, i.e. the neutral Spiro-OMeTAD-NH radical (Spiro-OMeTAD-NH●, Figure 2b). In particular, the computed UV-Vis spectrum of Spiro-OMeTAD-NH● shows a pronounced spectral feature at 580 nm, which can be related to the experimental feature found at 575 nm. It notably appears in a spectral region where the possible species that could be produced along with Spiro-OMeTAD-



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NH● are lacking any significant absorption. We can therefore assign the weak experimental absorption band at 575 nm (Figure 1d) to Spiro-OMeTAD-NH●. In order to probe the paramagnetic species generated by the PIL-doping, we performed electron paramagnetic resonance (EPR) on neat and PIL-doped Spiro-OMeTAD, as described in the Experimental Section. The EPR spectrum of the undoped Spiro-OMeTAD (see Supporting Information) shows a weak, symmetric signal due the presence of a small amount of SpiroOMeTAD+, which is predictably generated by exposure to oxygen.33,34 Otherwise, the spectrum of the PIL-doped Spiro-OMeTAD is more intense and slightly asymmetric, which suggests the superposition of multiple bands due to various paramagnetic species (i.e. Spiro-OMeTAD+ and Spiro-OMeTAD-NH●).42

Furthermore, the amplitude of the EPR spectra as a function of

microwave power, shown in the insert of Figure 2c, reveals a slow decrease of signal amplitude after the maximum value is reached. This heterogeneity in the power saturation profile is compatible with the presence of additional radical species together with Spiro-OMeTAD+. Indeed, protonated organic cations (Spiro-OMeTAD-NH+) can promote a single electron transfer from the parental neutral molecule (Spiro-OMeTAD) to the protonated molecule via radical formation (Spiro-OMeTAD+ and Spiro-OMeTAD-NH●, Figure 2d).40,43 We highlight here that in this particular system, the electron transfer is not spontaneous, as it needs to be thermally activated, described in the Experimental Section. ●

OMeTAD-NH

Then, even if we can still probe Spiro-

in the OS film (absorption at 575 nm in Figure 1d), this species could

spontaneously react to release hydrogen gas and regenerate neutral Spiro-OMeTAD.11 In Figure 2d, we summarize in a scheme the proposed PIL-doping mechanism. Notably, the mechanism requires two interacting Spiro-OMeTAD molecules. PIL-doping application in dye-sensitized solar cells. To demonstrate the applicability of the PIL-doping mechanism, we prepared ss-DSSCs and perovskite-based MSSCs utilizing SpiroOMeTAD as the organic hole transporting layer. The state-of-the art ss-DSSCs and MSSCs are assembled using lithium bis(trifluoromethylsulfonyl)-imide (Li-TFSI) and 4-tert-butylpyridine (tBP) as additives to Spiro-OMeTAD.13,44,45 The lithium ion (Li+) has been reported as essential for device operation since it helps lower the device series resistance (Rs) and enhances photocurrent generation.46,47 We recently demonstrated that the lowering of the Rs is mainly due to an effective p-doping of the organic hole transporter, which can be activated by Li+ only in presence of oxygen (Li-doping).34,48 However, we also showed that despite the Li-doping, the device power conversion efficiency is still significantly limited by Rs.34 Indeed, Burschka and co-workers were able to significantly enhance the device performance by simply introducing another p-dopant, in addition to Li-TFSI, to improve the conductivity of the hole transporting


Page 10 of 32

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


layer.13

Here, we demonstrate that replacing Li-TFSI with PIL-doping in Spiro-OMeTAD

decreases Rs, thus improving PCE. We first focus on ss-DSSCs, where the additives (Li-TFSI and tBP; or PILs) must be carefully investigated, since they affect not only the charge transport in the hole transporter but also the electron transport in the TiO2, as well as the charge recombination at TiO2/Spiro-OMeTAD interface. The ss-DSSCs have been prepared according to the procedure reported in the Experimental Section, using D102 dye to sensitized the TiO2.49,50

Figure 3. Charge transport, recombination, collection efficiency, and capacitance from photovoltage-current decay measurements51 for devices prepared with PILs and Li-TFSI. (a) Transport lifetimes at short circuit conditions (τtrans) against charge density at short circuit. (b) Recombination lifetimes at open circuit conditions (τrec) against charge density at open circuit. (c) Charge collection efficiency (ηcol) against charge density at short circuit. (d) Open circuit voltage against the differential capacitance. To quantify the influence of PILs on the device operation, we performed transient photovoltage photocurrent decay measurements on PILs and Li-TFSI based ss-DSSCs.51

From these

measurements, it is possible to extract the capacitance in the TiO2, recombination lifetime (τrec), charge transport lifetime (τtrans), and charge collection efficiency (ηcol).51,52 In Figure 3a we show that τtrans monotonically decreases with charge density for Li-TFSI and H-TFSI while Himi-TFSI and Et4N-TFSI seem to limit the charge transport at high charge density. To explain this trend,



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

we should consider that at low charge densities (about 1E+16 cm-3), the electron transport in the TiO2, rather than hole transport in the Spiro-OMeTAD, is limiting charge transport, this effect being similar for each dopant used.

53

As the charge density increases (above 1E+17 cm-3), the

electron transport rate in the TiO2 becomes comparable to the hole transport in the SpiroOMeTAD.54 In this regime, the hole transport can limit the charge transport if the SpiroOMeTAD is insufficiently doped.34,55 Therefore, Himi-TFSI and Et4N-TFSI are, effectively, limiting the charge transport for charge densities above 1E+17 cm-3, which is the standard operating range for devices.34 In Figure 3b we show that τrec follows the same trend with charge density for each dopant. At the same charge density, τrec increases as the cation hydrodynamic volume decreases (Et4N+ > Himi+ > Li+ > H+).56 A similar effect has been previously observed for liquid electrolyte DSSCs.57-59 The recombination trend with the cation volume can be explained by the relative ability of different positive ions to penetrate the dye layer and approach the TiO2 surface.46,60 Though the volumes of Et4N+ and Himi+ are comparable (depending on the surrounding molecules), Himi+ is expected to be considerably more compatible with the hydrophilic TiO2 surface than Et4N+.27 From the measurement of τtrans and τrec, we can extract the ηcol as the ratio of the transport rate constant (inverse of lifetime) over the sum of transport and recombination rate constants.52 Figure 3c shows ηcol remains constant with increasing charge density for Li-TFSI and H-TFSI, indicating that the faster charge transport compensates for the increased recombination as charge density increases. For Himi-TFSI and Et4N-TFSI, the same increase in recombination is not compensated by faster transport, which thus limits ηcol at higher light intensities. In Figure 3d we show the open circuit voltage (Voc) as a function of the differential capacitance in the TiO2.51 All the curves follow a similar trend, which suggests that the dopants used in this work leave the density of states distribution below the TiO2 conduction band unchanged.34,46,61-63 Therefore, any differences in Voc may be directly related to a shift in the relative position between the quasi-Fermi level for the electrons in TiO2 and the holes in Spiro-OMeTAD. Cappel and co-workers demonstrated that this energy alignment depends on the dipole moment augmented at the TiO2 interface.64 In our system, the interfacial dipole may be affected by the ability of the positive ion to penetrate the dye layer and approach the TiO2. Indeed, at the same capacitance, Voc decreases as the cation hydrodynamic volume decreases (Et4N+ > Himi+ > Li+ > H+). To demonstrate the impact of these parameters on power conversion efficiency, we report in Figure 4 the characteristic current-voltage (JV) curves for the same devices. Each characteristic has been measured under AM1.5 simulated sun light of 100 mW cm-2 equivalent solar irradiance with shadow masking as described in the Experimental Section.65 Table 1 lists the performance


Page 12 of 32

Page 13 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


parameters for the devices reported in Figure 4 and the device series resistance as extracted by fitting JV curves to an ideal diode model.34

Figure 4. Photocurrent-voltage curves for devices employing different additives. All devices were sensitized with D10249,50 and measured under AM1.5 simulated sun light of 100 mW cm -2 equivalent solar irradiance. All devices employed the same amount of tBP as described in the Experimental Section. Table 1. Solar cell performance parameters: short circuit photocurrent (Jsc), power conversion efficiency (PCE), open circuit voltage (Voc), fill factor (ff), and series resistances (Rs), as extracted by fitting JV curves to an ideal diode model.34

H-TFSI Li-TFSI Himi-TFSI Et4N-TFSI

Jsc (mA cm-2) 7.53 5.70 4.77 2.44

PCE (%) 3.34 3.26 2.26 1.28

Voc (V) 0.70 0.74 0.86 0.96

ff 0.62 0.75 0.54 0.54

Rs (Ohm) 43 57 224 1293

The short circuit photocurrent density (Jsc) shows monotonic growth with cation size from Et4NTFSI to H-TFSI. This trend could be explained considering the shift of the electron Fermi level in the TiO2 for different additives as described in Figure 3d. As widely reported in literature, a downshift of the Fermi level in the TiO2 results in a more favourable electron injection from the dye excited state to the metal oxide.66,67 The Jsc for Li-TFSI, according to the trend observed in Figure 3d, is located between Himi-TFSI and H-TFSI. For the same reason, open circuit voltage (Voc) follows the opposite trend. The fill factor trend could be explained by considering both the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

charge recombination and the device series resistance due to the hole transport in the SpiroOMeTAD.53 For Li-TFSI and H-TFSI, the device series resistance are similar (see Table 1) and the recombination lifetime at the same charge density is longer for H-TFSI (see Figure 3b), which could suggest a better fill factor for the latter. However, even if at same charge density Li-TFSI is recombining faster, H-TFSI can reach four times higher charge density under 100 mW cm -2 simulated sun light (4E+17 cm-3) than Li-TFSI (1E+17 cm-3). Himi-TFSI and Et4N-TFSI suffer from both high recombination and series resistance, resulting in significantly diminished fill factor. The overall PCE shows that H-TFSI and Li-TFSI perform similarly whileHimi-TFSI and Et4N-TFSI perform sub-standardly. We note here that the previous devices have been prepared, according to the state-of-art ssDSSCSs, employing tBP as an additive together with PILs or Li-TFSI, as described in the Experimental Section.13,45,68 The tBP has been reported to raise the surface potential of the TiO2, which partially compensates for the positive shift of Li+ by recovering the Voc and decreasing the Jsc.58 The highest PCE for Li-TFSI-based deviceshas been achieved by tuning the relative amount of Li+ and tBP in order to attain a well-balanced compromise between the Jsc and Voc.34 In the aforementioned experiment, we used additives, both the PILs and the tBP, at the same concentration that has been optimized for Li-based devices. However, focusing on Himi-TFSI in particular, the substantially reduced Jsc, despite high Voc, suggests that the addition of tBP could be detrimental for its performance. In Figure 5, we reported the device JV for Himi-TFSI without tBP compared to the optimized Li-TFSI tBP one. Notably, the Himi-TFSI reaches 4.6% efficiency, which surpasses the previous record reported for D102-based ss-DSSCs.49,50 We should note that immediately after the preparation Himi-TFSI (without tBP) performs very close to traditional Li-TFSI tBP devices (see Figure 4). Then, after storing for one month the devices in air and dark, they showed the JV curves reported in Figure 5. This procedure allows the conductivity of the Himi-doped Spiro-OMeTAD to increase considerably due to the doping effect ofoxygen.69 Indeed, the series resistances after one month, reported in Table 2, show similar values for Li-TFSI and Himi-TFSI.


Page 14 of 32

Page 15 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Photocurrent-voltage curves reported for devices employing the optimized additives composition Li-TFSI and tBP; or Himi-TFSI. The devices were sensitized with D10249,50 and measured under AM1.5 simulated sun light of 100 mW cm -2 equivalent solar irradiance after one month storarage in air and dark. Table 2. Solar cell performance parameters: short circuit photocurrent (Jsc), power conversion efficiency (PCE), open circuit voltage (Voc), fill factor (ff) and series resistances (Rs), as extracted by fitting JV curves to an ideal diode model.34

Himi-TFSI (tBP omitted) Li-TFSI + tBP

Jsc (mA cm-2)

PCE (%)

Voc (V)

ff

Rs (Ohm)

7.63

4.66

0.82

0.64

87

6.31

3.18

0.79

0.64

89

To elucidate the role of the Himi-TFSI as a replacement for Li-TFSI and tBP, we performed photoinduced absorption spectroscopy (PIA) along with transient absorbance spectroscopy (TAS).70



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. (a) Photoinduced absorption spectra (PIA) for D10249,50 adsorbed on TiO2 with and without the addition of Li-TFSI, tBP and Himi-TFSI. (b) PIA for D102 after addition of the hole transporter with and without the addition of Li-TFSI, tBP and Himi-TFSI. Inset, lifetime collected from transient absorption spectra (TAS) at 560 nm and 1600 nm on the same samples (the spectra are reported in Supporting Information). In Figure 6a, we reported the PIA for the dye (D102)49,50 adsorbed onto TiO2 in the presence and absence of Li-TFSI/ tBP and Himi-TFSI. We observe the negative feature at 570 nm due to the Stark effect71 and two positive features at 720 and 1160 nm, which have been attributed to the oxidized dye.70 The Stark effect is thought to arise from an electric filed generated by the buildup of electrons in the TiO2 and holes on the dye or on the Spiro-OMeTAD.70 In essence, a


Page 16 of 32

Page 17 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


stronger Stark signal suggests less effective charge screening at the TiO2 interface.72 After normalizing the curves at the same concentration of holes on the dye (1160 nm), we note a significant difference in the Stark signal amplitude between the Himi+ and Li+ (Li+ with and without tBP). This could be explained by considering the diminished ability of the bulkier Himi cation to penetrate the dye layer and screen the field induced by the separated electrons and holes across the dye-sensitized interface.57-59 In Figure 6b, we reported the same spectra collected after addition of Spiro-OMeTAD. We still observe a Stark effect at 560 nm and the two positive features at 700 and 1600 nm (the latter being broad), which in this case may be attributed to oxidized Spiro-OMeTAD.47 After normalizing the spectra at the same concentration of holes on Spiro-OMeTAD (1600 nm), we can see that the relative amplitudes of the Stark signal (560 nm) is higher for the Li-TFSI/tBP system, moderate for both the Himi-TFSI system and for neat Spiro-OMeTAD, and reduced to almost zero for Li-TFSI only. This trend is different from what we observed without Spiro-OMeTAD (Figure 6a), where the Himi+ was screened charges less than Li+. To explain this difference, we must consider that Li cations in Spiro-OMeTAD film are consumed in presence of oxygen.34,47 After the addition of Spiro-OMeTAD, their effective concentration at the TiO2 interface could be significantly lower than the optimized one. In addition, the tBP could further reduce the concentration of Li cations on the TiO2 surface with two possible mechanisms: binding the cation via Li+ /π complexes interaction, organic layer on the TiO2 surface,

74

73

or creating an +

which could reduce the ability of residual Li intercalating

into the metal oxide and thereby screening the injected charges. Although Himi exhibited a stronger Stark signal than Li+, after the addition of tBP, the situation is reversed. Using TAS, we can monitor the lifetime of the Stark features at 560 nm (τ560), as reported in table inset Figure 6b. τ560 represents the kinetic of the screening reaction for the photogenerated charges at TiO2 interface.71,72 We can see that Himi is screening charges faster (114 µs) than the Li tBP (146 µs). Again, Li alone could screen the charge much faster (only 12 µs), but tBP is always needed to optimize the device performance, as previously discussed. Similarly, TAS at 1600 nm allows direct monitoring of the lifetime for the photogenerated hole in the SpiroOMeTAD (τ1600). From the data reported in table inset Figure 6b, we can see that the holes recombine faster (shorter lifetime) for Himi+ than for Li+/tBP.

This can be explained by

considering the recombination-blocking effect of the tBP adsorbed onto TiO2 surface.44 PIL-doping application in meso-superstructured solar cells. Lastly, we demonstrate the application of PIL-doped Spiro-OMeTAD in MSSCs.25

In this device structure the

mesoporous TiO2 is replaced by an insulating dielectric such as Al2O3. The latter mesoporous



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 32

metal oxide acts only as scaffold to assist in the film formation and crystallization of the organometal halide perovskite absorber (see inset Figure 7), which generates and transports charge.26

In this device, a thin compact layer of TiO2 (thickness 75 nm, see Supporting

Information) is nevertheless used as a selective contact for the electrons at the bottom of the active layer (thickness 400 nm, see Supporting Information).25 Although we cannot completely exclude a direct contact between the Spiro-OMeTAD and the compact TiO2 layer,26 the occurrence of this interface is considerably reduced compared to ss-DSSCs. This should allow the use of H-TFSI rather than Himi-TFSI, as a more effective PIL-dopant without significant loss in Voc, the latter of which was detrimental to maximizing the performance of ss-DSSCs (Figure 4).

Figure 7. Photocurrent-voltage curves for devices employing Li-TFSI or H-TFSI. The devices were prepared using perovskite crystals ABX3 (A = CH3NH3, B = Pb, and X = I3-x, Clx)25 as shown and measured under AM1.5 simulated sun light of 100 mW cm -2 equivalent solar irradiance. Table 3. Solar cell performance parameters: short circuit photocurrent (Jsc), power conversion efficiency (PCE), open circuit voltage (Voc), fill factor (ff) and series resistances (Rs), as extracted by fitting JV curves to an ideal diode model.34

H-TFSI Li-TFSI

Jsc (mA cm-2) 18.32 18.29

PCE (%) 12.08 11.28

Voc (V) 1.08 1.06

ff 0.60 0.56

Rs (Ohm) 87 120

In Figure 7, we reported the device JV curves for devices using Spiro-OMeTAD doped with HTFSI or Li-TFSI.26 As reported in Table 2, PIL-doped devices show higher ff and similar Voc and Jsc compared to Li-doped devices. Since we could not find any significant difference both in


Page 19 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PIA and TAS, we conclude that the origin of improved ff can be directly correlated to a reduced series resistance, which can be achieved by utilizing PIL-doping of the organic semiconductor.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CONCLUSIONS In summary, we have reported a new p-doping mechanism, which is compatible with solution processing of organic semiconductors in air. The conductivity of OS films significantly improves if PILs are added in the precursor solution. In particular, we show that H-TFSI increases the conductivity of Spiro-OMeTAD by three orders of magnitude. We demonstrate that H-TFSI promotes Spiro-OMeTAD single electron oxidation and thus effective p-doping in the semiconducting film.

We describe a general PIL-doping mechanism that “consumes”

electrons, leaving free holes on the organic matrix. Although in this work we report data only for Spiro-OMeTAD, we have observed PIL-doping in poly(3-hexylthiophene), poly (N,Ndiphenyl-4-methoxyphenylamine-4′,4′′-diyl) and sexthiophene (more details to follow in a subsequent publication). Exemplifying these concepts, we demonstrate hybrid solar cells, both ss-DSSCs and MSSCs, with improved PCE compared to the state-of-the-art. Particularly, we replace the commonly used additive composition, Li-TFSI and tBP, with a single protic ionic liquid. We use photovoltage - photocurrent decay and photoinduced absorption spectroscopy to establish that the significantly improved device performance is attributed to reduced charge transport resistance in the hole-transporting matrix potentiated by PIL-doping.


Page 20 of 32

Page 21 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


EXPERIMENTAL SECTION

Doping procedure. The doping procedure was carried out under inert atmosphere (oxygen and water free). Chlorobenzene and acetonitrile were purchased from Sigma-Aldrich in reagent and anhydrous grade. Before introducing the solvents into an oxygen- and water- free glovebox environment, they were deaerating by bubbling with dry nitrogen for several hours. SpiroOMeTAD was purchased from Lumetec and the dopants (H-TFSI, Li-TFSI, Himi-TFSI, and Et4N-TFSI) from Sigma-Aldrich. All the chemicals were purchased in their highest available purity and they were used without further purification. They have been delivered sealed in inert atmosphere and they were opened and stored directly in a glovebox. In particular, H-TFSI is highly sensitive to the air moisture and just a few minutes of air exposure could significantly hydrate it. Then, a 90 mM solution of Spiro-OMeTAD in chlorobenzene was prepared by stirring the mixture overnight at 70°C. Solutions of ionic liquids (H-TFSI, Himi-TFSI and Et4NTFSI) and Li-TFSI in acetonitrile have been prepared by varying the concentration in order to add always the same volume (1/10 of the final volume) of dopant solution (in acetonitrile) to the Spiro-OMeTAD solution (in chlorobenzene).

The solution Spiro-OMeTAD plus dopant

(chlorobenzene /acetonitrile 1/10 v/v) was left stirring at 120°C overnight in a glovebox. After cooling to room temperature, the solution was filtered using a PTFE membrane of pore size 0.2 µm (Aldrich). Where appropriate, tBP was added (10 µL each mL of solution) immediately before spin-coating the solution onto the substrate.

Conductivity measurements. Spiro-OMeTAD solutions with various amounts of dopant were spin-coated at 2000 rpm onto a mesoporous SiO2 layer prepared on glass slides (particle size 40 nm, mesoporous layer thickness 300 nm) and were dried at 140 °C for 30 min in inert atmosphere. Silver contacts (150 nm thick) were applied via thermal evaporation through a shadow mask in 2×10-6 Torr vacuum. The electrode pattern was designed for four point probe measurements with a force channel length (direction of current flow) of 1 mm and a width of 1 cm and a sense channel length of 300 µm and width of 1 mm. The J-V characteristics of these samples were collected at room temperature with a Keithley 2420 Source Meter unit to extract both the bulk and the contact resistance.75 The contact resistance was found to be negligible compared to the bulk at each doping level. All the samples were prepared and measured under inert atmosphere and with filtered UV light.

No relevant visible light dependence of

conductivity was observed.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

UV-Vis spectra. The sample preparation was identical to the devices for conductivity measurements with the exception that no silver electrodes were deposited on the top of the sample. All the samples were prepared in inert atmosphere and in filtered UV light. Absorbance measurements of the films were taken in air (approximately 1 min air exposure) using a commercial spectrophotometer (Varian Cary 300 UV- Vis, USA). Baseline measurements were performed on mesoporous SiO2 substrate without Spiro-OMeTAD. Nuclear magnetic resonance. spectrometer (500 MHz).

1

H NMR spectra were recorded on a Bruker Advance 500

Anhydrous deuterated dimethyl sulfoxide was used as solvent;

chemical shifts, δ, are given in ppm, using the solvent residual signal as the internal standard. The samples were prepared in inert atmosphere and the spectra were collected immediately after the preparation. A complete signal quenching of Spiro-OMeTAD was observed after a solution of Spiro-OMeTAD and Himi-TFSI was left for approximately 3 hours.

Himi-TFSI: 8.75 (s, 1H); 7.32 (t, J= 1.7 Hz, 1H); 7.29 (t, J= 1.7 Hz, 1H); 4.20-4.12 (m, 2H); 3.94 (s, 3H); 1.93-1.78 (m ,2H); 1.38-1.22 (m, 6H); 0.88 (t, J=6.9 Hz, 3H).

Spiro-OMeTAD: 7.46 (d, J=8.4 Hz, 4H); 6.91-6.73 (m, 32H); 6.69 (d, J=1.6 Hz, 2H); 6.67 (d, J=1.7 Hz, 2H); 6.17 (d, J=1.9 Hz, 4H); 3.70 (s, 24H).

Spiro-OMeTAD + Himi-TFSI (30 mol %): 9.66 (s, 1H); 7.40-7.19 (m, 4H); 6.95-6.87 (m, 0.1H); 6.85-6.75 (m, 0.5H); 4.24 (t, J=7.4 Hz, 2H); 4.03 (s, 3H), 3.80-3.75 (m, 1H); 1.96-1.75 (m ,?H); 1.39-1.21 (m, 6H); 0.89 (t, J=6.7 Hz, 3H). Table 4. 1H NMR chemical shift of significant peaks. Himi-TFSI 8.75 (s, 1H)

Spiro-OMeTAD 6.91-6.73 (m, 32H) 6.69 (d, J=1.6 Hz, 2H); 6.67 (d, J=1.7 Hz, 2H)

4.20-4.12 (m, 2H) 3.94 (s, 3H) 3.70 (s, 24H) 1.93-1.78 (m ,2H) 1.38-1.22 (m, 6H) 0.88 (t, J=6.9 Hz, 3H)

Spiro-OMeTAD + Himi-TFSI (30 mol %) 9.66 (s, 1H); 6.95-6.87 (m, 0.1H) 6.85-6.75 (m, 0.5H) 4.24 (t, J=7.4 Hz, 2H) 4.03 (s, 3H) 3.80-3.75 (m, 1H) 1.96-1.75 (m) 1.39-1.21 (m, 6H) 0.89 (t, J=6.7 Hz, 3H)


Page 22 of 32

Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Computational details.

The molecular structure of both the protonated and the non-

protonated species was optimized using the B3LYP76-78 as the exchange-correlation functional and 6-31G** as the basis set79,80 both in vacuum and in solution by including the solution effects through a conductor-like polarizable continuum model (C-PCM)81-84 with ethanol as the solvent. We used the same computational level employed in a previous report by Fantacci et al.,33 even though in the present study all the reported calculations were performed using the Gaussian 09 program package.33 The electronic structure of the investigated species was briefly analyzed. To evaluate the oxidation potential of the investigated species, we used the rigorous calculation of the Gibbs free energy difference between the neutral and oxidized species (see the report from Oligaro et al.85 for details); concisely, the Gibbs free energy in solution (Gisol) of the oxidized and the reduced species are computed by adding the Gibbs energy in vacuum (Givac) (obtained by including the vibrational contribution to the partition function) to the energy of solvation of the given species (∆Gsolv). Electron paramagnetic resonance (EPR) analysis. The sample was measured using an X-band (9 GHz) Bruker Elexys E-500 spectrometer (Bruker, Rheinstetten, Germany). Capillaries containing the samples were placed in a standard 4 mm quartz sample tube. The temperature of the sample was regulated to and maintained at 25 °C throughout the measurement by passing thermostatic nitrogen gas through a quartz Dewar. The instrument settings used were sweep width, 100 G; resolution, 1024 points; modulation frequency, 100 kHz; modulation amplitude, 1.0 G; time constant, 20.5 ms. The EPR spectrum was measured with an attenuation of 10 dB (approximately 20 mW) to avoid microwave saturation of the resonance absorption curve. Numerous scans, typically 16, were accumulated to improve the signal-to-noise ratio. The spectral parameters calculated from this averaged spectrum are summarized in Table 5.

Table 5. EPR spectral parameters for Spiro-OMeTAd doped with H-TFSI.

Spiro-OMeTAD – H-TFSI g-factor 2.0032±0.0004 ∆B(G) 4.6 ±0.2 Spin g-1 8.4x106 The g-factor was determined by using an internal standard to provide an exact position of the EPR signal.

The standard was an ethanol solution of 4-Hydroxy-TEMPO.42


This solution was


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 32

introduced into the EPR tube, where a capillary containing Spiro-OMeTAD powder was also added in order to measure the corresponding spectrum.

Solid-State Dye-Sensitized Solar Cells (ss-DSSCs) fabrication. ss-DSSCs were prepared according to the previous method.34 All solvents used for device fabrication were reagentand anhydrous grade. FTO substrates (7 Ω/sq, Pilkington, US) were etched with zinc powder and 2 M aqueous HCl to give the desired electrode patterning. The substrates were cleaned with Hellmanex (2% by volume in water, Aldrich), de-ionized water, acetone, and ethanol. Remaining organic residues were removed with10 minutes of oxygen plasma cleaning. The FTO sheets were subsequently coated with a compact layer of TiO2 (about 30 nm) by aerosol spray pyrolysis deposition at 275 °C, using oxygen as the carrier gas. Films of 1.5 µm thick mesoporous TiO2 were then deposited by screen-printing a commercial paste (Dyesol 18NR-T). The TiO2 films were gradually heated to 500 °C and were sintered for 30 min in air. Once cooled, the samples were immersed into a 15 mM TiCl4 aqueous solution for 45 min at 70 °C and then sintered at 500 °C for 45 min. After cooling to 70 °C, the substrates were immersed in a 500 µM dye solution, in a 1:1 mixture of acetonitrile and tert-butyl alcohol by volume, for one hour. The dye used in this study was D102, previously reported by Horiushi et al.49 After the dyed films were rinsed in acetonitrile, the Spiro-OMeTAD was applied by spin-coating at 1000 rpm for 45 s in air from a solution prepared as previously described. After drying overnight, 150 nm silver back electrode contacts were applied by thermal evaporation .

Meso-Superstructures Solar Cells (MSSCs) fabrication.

MSSCs were prepared

according to the previous method.26 All solvents used for device fabrication were reagent and anhydrous grade. FTO substrates (7 Ω/sq, Pilkington US) were etched with zinc powder and 2 M aqueous HCl to give the desired electrode patterning. The substrates were cleaned with Hellmanex (2% by volume in water, Aldrich), de-ionized water, acetone, and ethanol. Remaining organic residues were removed with10 minutes of oxygen plasma cleaning.The FTO sheets were subsequently coated with a compact layer of TiO2 (approximately 30 nm) by aerosol spray pyrolysis deposition at 270 °C, using oxygen as the carrier gas. The TiO2 films were gradually heated to 500 °C and sintered for 30 min in air. Once cooled, 200 nm thick films of mesoporous Al2O3 were deposited on the top of the compact TiO2 layer by spin-coating an Aldrich isopropanol dispersion of 40 nm Al2O3 particles. The substrates were then heated to 150 °C for 30 min and then cooled to room temperature. Then, perovskite crystals ABX3 (A = CH3NH3, B = Pb, and X = I1-x, Clx) were deposited according to the procedure previously


Page 25 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


reported.25 The Spiro-OMeTAD was applied by spin-coating at 2000 rpm for 45 s in air from a solution prepared as previously described. After drying overnight, 150 nm silver back electrode contacts were applied by thermal evaporation .

Photovoltage and photocurrent decay measurements were performed by a similar method to O’Regan et al. and as described elsewhere.61,86,87 All devices were stored in air and in dark for 72 hrs prior to testing.

Photoinduced Absorption Spectroscopy (PIA). The films were excited withmonochromatic light (496.5 nm) at a power density of 50 mW cm-2, modulated with a chopper at 23 Hz. White light from a halogen lamp was used as a probe. After passing through a monochromator (Princeton Instruments, SP2300), the signal was detected with an amplified Si diode in the visible (Thorlabs, PDA10A) and InGaAs (Acton Research Coproration, ID-441-C) in the nearinfrared. Signals were acquired with a lock-in (Stanford Research Systems, SRS830) and an acquisition module (National instruments, USB-6008). The instrument was controlled with National Instruments LabView software. Samples were prepared according to the fabrication procedure described for ss-DSSCs in the previous paragraph.

Transient Absorption Spectroscopy (TAS). Pulsed excitation (λex = 500 nm, 7 ns pulse duration, 10 Hz repetition rate) of the dyed film was performed using an Nd:YAG laser (Ekspla, NT340) with a laser fluence on the sample maintained at less than 50 µJ cm-2 per pulse. The probe light, generated by a continuous-wave xenon arc lamp, was first passed through a monochromator (Acton Research Corporation, SpectraPro- 2150i), various optical elements, the sample, and then through a second monochromator (Acton Research Corporation, SpectraPro2300i) before being detected in the visible region by a fast photomultiplier tube module biased with 750 V (Hamamatsu, R9110).

In the NIR, detection utilized a fast diode (Thorlabs,

SM05PD5A). Signals were recorded on an oscilloscope (Tektronix, DPO3054). Satisfactory signal-to-noise ratios were typically obtained by averaging 256 laser pulses. The data was smoothed with a Savitsky–Golay algorithm for presentation. Samples were prepared according to the fabrication procedure described for ss-DSSCs in the previous paragraph.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Photovoltaic characterization.65 For measuring device merit parameters, simulated AM 1.5 sunlight was generated with a class AAB ABET solar simulator calibrated to AM1.5, 100 mWcm-2 irradiance, using an NREL-calibrated KG5 filtered silicon reference cell, with less than 1% mismatch factor; the current-voltage curves were recorded with a sourcemeter (Keithley 2400, USA). The solar cells were masked with a metal mask defining the active area (0.08 cm2) of the solar cells.

MSSCs were measured immediately after the thermal evaporation of

electrodes.


Page 26 of 32

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgment This work was funded by the Engineering and Physical Sciences Research Council (EPSRC) APEX project and the European Commission, under the SANS project, grant agreement number 246124.

Supporting Information Himi-TFSI doped Spiro-OMeTAD UV-Vis absorption; Spiro-OMeTAD 1H-NMR in deuterated dimethyl sulfoxide; optimized structures of Spiro-OMeTAD protonated species and relative formation energies; EPR spectrum of undoped Spiro-OMeTAD; transient absorption spectra; device cross-section scanning electron microscopy image. This information is available free of charge via the internet at http://pubs.acs.org.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

References (1) Mei, J.; Diao, Y.; Appleton, A. L.; Fang, L.; Bao, Z. Journal of the American Chemical Society 2013, 135, 6724. (2) Chiang, C.; Fincher Jr, C.; Park, Y.; Heeger, A.; Shirakawa, H.; Louis, E.; Gau, S.; MacDiarmid, A. G. Physical Review Letters 1977, 39, 1098. (3) MacDiarmid, A. G. Angewandte Chemie International Edition 2001, 40, 2581. (4) MacDiarmid, A. G.; Heeger, A. J. Synthetic metals 1980, 1, 101. (5) Han, C.; Elsenbaumer, R. Synthetic metals 1989, 30, 123. (6) dos Santos, D.; Galvão, D.; Laks, B. Solid State Communications 1990, 74, 215. (7) Spangler, C. W.; Bryson, P.; Liu, P. K.; Dalton, L. R. J. Chem. Soc., Chem. Commun. 1992, 253. (8) Hu, B.; Zhu, X.; Chen, X.; Pan, L.; Peng, S.; Wu, Y.; Shang, J.; Liu, G.; Yan, Q.; Li, R. W. Journal of the American Chemical Society 2012, 134 (42),17408–17411. (9) Nam, S.; Kim, J.; Lee, H.; Kim, H.; Ha, C.-S.; Kim, Y. ACS applied materials & interfaces 2012, 4, 1281. (10) Zhu, X. Q.; Zhang, M. T.; Yu, A.; Wang, C. H.; Cheng, J. P. Journal of the American Chemical Society 2008, 130, 2501. (11) Wei, P.; Oh, J. H.; Dong, G.; Bao, Z. Journal of the American Chemical Society 2010, 132, 8852. (12) Wren, S. W.; Vogelhuber, K. M.; Garver, J. M.; Kato, S.; Sheps, L.; Bierbaum, V. M.; Lineberger, W. C. Journal of the American Chemical Society 2012, 134, 6584. (13) Burschka, J.; Dualeh, A.; Kessler, F.; Baranoff, E.; Cevey-Ha, N. L.; Yi, C.; Nazeeruddin, M. K.; Grätzel, M. Journal of the American Chemical Society 2011, 133 (45), 18042–18045. (14) DAndrade, B. W.; Forrest, S. R.; Chwang, A. B. Applied physics letters 2003, 83, 3858. (15) Pfeiffer, M.; Leo, K.; Zhou, X.; Huang, J. S.; Hofmann, M.; Werner, A.; Blochwitz-Nimoth, J. Organic Electronics 2003, 4, 89. (16) Leijtens, T.; Ding, I. K.; Giovenzana, T.; Bloking, J. T.; McGehee, M. D.; Sellinger, A. ACS nano 2012, 6 (2), 1455–1462. (17) Snaith, H. J.; Zakeeruddin, S. M.; Wang, Q.; Péchy, P.; Grätzel, M. Nano Letters 2006, 6, 2000. (18) Shen, Y.; Hosseini, A. R.; Wong, M. H.; Malliaras, G. G. ChemPhysChem 2004, 5, 16. (19) Kruger, J.; Plass, R.; Cevey, L.; Piccirelli, M.; Gratzel, M.; Bach, U. Applied Physics Letters 2001, 79, 2085. (20) Blochwitz, J.; Fritz, T.; Pfeiffer, M.; Leo, K.; Alloway, D. M.; Lee, P. A.; Armstrong, N. R. Organic Electronics 2001, 2, 97. (21) Riede, M.; Uhrich, C.; Widmer, J.; Timmreck, R.; Wynands, D.; Schwartz, G.; Gnehr, W. M.; Hildebrandt, D.; Weiss, A.; Hwang, J. Advanced Functional Materials 2011, 21, 3019–3028. (22) Greaves, T. L.; Drummond, C. J. Chemical reviews 2008, 108, 206. (23) Margaretta, E.; Olmeda, C.; Yu, L. Journal of Applied Polymer Science 2013, 127, 2453. (24) Bach, U.; Lupo, D.; Comte, P.; Moser, J.; Weissörtel, F.; Salbeck, J.; Spreitzer, H.; Grätzel, M. Nature 1998, 395, 583. (25) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Science 2012, 338, 643.


Page 28 of 32

Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(26) Ball, J. M.; Lee, M. M.; Hey, A.; Snaith, H. Energy & Environmental Science 2013, 6, 1739–1743. (27) Abate, A.; Petrozza, A.; Roiati, V.; Guarnera, S.; Snaith, H.; Matteucci, F.; Lanzani, G.; Metrangolo, P.; Resnati, G. Organic Electronics 2012, 13 (11), 2474–2478. (28) Abate, A.; Petrozza, A.; Cavallo, G.; Lanzani, G.; Matteucci, F.; Bruce, D. W.; Houbenov, N.; Metrangolo, P.; Resnati, G. J. Mater. Chem. A 2013, 1, 6572. (29) Armand, M.; Endres, F.; MacFarlane, D. R.; Ohno, H.; Scrosati, B. Nature materials 2009, 8, 621. (30) Yu, L.; Pizio, B. S.; Vaden, T. D. J. Phys. Chem. B 2012, 116, 6553−6560. (31) Fevre, M. v.; Pinaud, J.; Leteneur, A.; Gnanou, Y.; Vignolle, J.; Taton, D.; Miqueu, K.; Sotiropoulos, J.-M. Journal of the American Chemical Society 2012, 134, 6776. (32) Ott, L. S.; Cline, M. L.; Deetlefs, M.; Seddon, K. R.; Finke, R. G. Journal of the American Chemical Society 2005, 127, 5758. (33) Fantacci, S.; De Angelis, F.; Nazeeruddin, M. K.; Grätzel, M. The Journal of Physical Chemistry C 2011, 91 (2), 192–198. (34) Abate, A.; Leijtens, T.; Pathak, S.; Teuscher, J.; Avolio, R.; Errico, M. E.; Kirkpatrik, J.; Ball, J. M.; Docampo, P.; McPherson, I. Physical Chemistry Chemical Physics 2013, 15, 2572. (35) Gregg, B. A.; Chen, S. G.; Cormier, R. A. Chemistry of materials 2004, 16, 4586. (36) Bertini, I.; Luchinat, C.; Parigi, G. Solution NMR of paramagnetic molecules: applications to metallobiomolecules and models; Elsevier Science, 2001; Vol. 2. (37) Yoder, J. C.; Roth, J. P.; Gussenhoven, E. M.; Larsen, A. S.; Mayer, J. M. Journal of the American Chemical Society 2003, 125, 2629. (38) Uğurbil, K.; Mitra, S. Proceedings of the National Academy of Sciences 1985, 82, 2039. (39) Jensen, M. R.; Hansen, D. F.; Led, J. J. Journal of the American Chemical Society 2002, 124, 4093. (40) Goez, M.; Frisch, I.; Sartorius, I. Beilstein Journal of Organic Chemistry 2013, 9, 437. (41) Lungwitz, R.; Spange, S. ChemPhysChem 2012, 13, 1910. (42) Alvarez, F. J. D.; Orelle, C.; Davidson, A. L. Journal of the American Chemical Society 2010, 132, 9513. (43) She, C.; Anderson, N. A.; Guo, J.; Liu, F.; Goh, W.-H.; Chen, D.-T.; Mohler, D. L.; Tian, Z.-Q.; Hupp, J. T.; Lian, T. The Journal of Physical Chemistry B 2005, 109, 19345. (44) Krüger, J.; Plass, R.; Cevey, L.; Piccirelli, M.; Grätzel, M.; Bach, U. Applied physics letters 2001, 79, 2085. (45) Katz, M. J.; Vermeer, M. J. D.; Farha, O. K.; Pellin, M. J.; Hupp, J. T. Langmuir 2012, 29 (2), 806–814. (46) Cappel, U. B.; Smeigh, A. L.; Plogmaker, S.; Johansson, E. M. J.; Rensmo, H.; Hammarstro m, L.; Hagfeldt, A.; Boschloo, G. The Journal of Physical Chemistry C 2011, 115 (10), 4345–4358. (47) Cappel, U. B.; Daeneke, T.; Bach, U. Nano Letters 2012, 12 (9), 4925–4931. (48) Li, Y.-F.; Selloni, A. Journal of the American Chemical Society 2013, 135 (24), 9195–9199. (49) Horiuchi, T.; Miura, H.; Uchida, S. Chemical Communications 2003, 3036. (50) Schmidt‐Mende, L.; Bach, U.; Humphry‐Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Grätzel, M. Advanced Materials 2005, 17, 813. (51) Barnes, P. R.; Miettunen, K.; Li, X.; Anderson, A. Y.; Bessho, T.; Gratzel, M.; O'Regan, B. C. Advanced Materials 2013, 25, 1881–1922.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(52) Sivaram, V.; Kirkpatrick, J.; Snaith, H. Journal of Applied Physics 2013, 113, 063709. (53) Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Bisquert, J. The Journal of Physical Chemistry B 2003, 107, 758. (54) Leijtens, T.; Lim, J.; Teuscher, J.; Park, T.; Snaith, H. J. Advanced Materials 2013, 25, 3227–3233. (55) Wang, H.; Peter, L. M. The Journal of Physical Chemistry C 2009, 113, 18125. (56) Schröder, C.; Wakai, C.; Weingärtner, H.; Steinhauser, O. The Journal of Chemical Physics 2007, 126, 084511. (57) Fredin, K.; Nissfolk, J.; Boschloo, G.; Hagfeldt, A. Journal of Electroanalytical Chemistry 2007, 609, 55. (58) Koops, S. E.; O’Regan, B. C.; Barnes, P. R. F.; Durrant, J. R. Journal of the American Chemical Society 2009, 131, 4808. (59) Pu, P.; Cachet, H.; Laidani, N.; Sutter, E. The Journal of Physical Chemistry C 2012, 116, 22139. (60) Abrusci, A.; Santosh Kumar, R. S.; Al‐Hashimi, M.; Heeney, M.; Petrozza, A.; Snaith, H. J. Advanced Functional Materials 2011, 21, 2571. (61) O'Regan, B.; Xiaoe, L.; Ghaddar, T. Energy & Environmental Science 2012, 5, 7203. (62) Kopidakis, N.; Benkstein, K. D.; van de Lagemaat, J.; Frank, A. J. The Journal of Physical Chemistry B 2003, 107, 11307. (63) Greiner, M. T.; Helander, M. G.; Tang, W. M.; Wang, Z. B.; Qiu, J.; Lu, Z. H. Nature materials 2011, 11, 76–81. (64) Cappel, U. B.; Plogmaker, S.; Johansson, E. M.; Hagfeldt, A.; Boschloo, G.; Rensmo, H. Physical Chemistry Chemical Physics 2011, 13, 14769. (65) Snaith, H. J. Energy & Environmental Science 2012, 5, 6513. (66) Kelly, C. A.; Farzad, F.; Thompson, D. W.; Stipkala, J. M.; Meyer, G. J. Langmuir 1999, 15, 7047. (67) Lunell, S.; Stashans, A.; Ojamäe, L.; Lindström, H.; Hagfeldt, A. Journal of the American Chemical Society 1997, 119, 7374. (68) Kim, J.-Y.; Kim, J. Y.; Lee, D.-K.; Kim, B.; Kim, H.; Ko, M. J. The Journal of Physical Chemistry C 2012, 116, 22759. (69) Nicolai, H.; Kuik, M.; Wetzelaer, G.; de Boer, B.; Campbell, C.; Risko, C.; Brédas, J.; Blom, P. Nature materials 2012, 11, 882. (70) Boschloo, G.; Hagfeldt, A. Inorganica Chimica Acta 2008, 361, 729. (71) Ardo, S.; Sun, Y.; Castellano, F. N.; Meyer, G. J. The Journal of Physical Chemistry B 2010, 114, 14596. (72) Ardo, S.; Sun, Y.; Staniszewski, A.; Castellano, F. N.; Meyer, G. J. Journal of the American Chemical Society 2010, 132, 6696. (73) Gal, J.-F.; Maria, P.-C.; Decouzon, M.; Mó, O.; Yáñez, M.; Abboud, J. L. M. Journal of the American Chemical Society 2003, 125, 10394. (74) Canesi, E. V.; Binda, M.; Abate, A.; Guarnera, S.; Moretti, L.; D'Innocenzo, V.; Kumar, R. S. S.; Bertarelli, C.; Abrusci, A.; Snaith, H. Energy & Environmental Science 2012, 5, 9068. (75) Planells, M.; Abate, A.; Hollman, D. J.; Stranks, S. D.; Bharti, V.; Gaur, J.; Mohanty, D.; Chand, S.; Snaith, H.; Robertson, N. J. Mater. Chem. A 2013, 1, 6949-6960. (76) Becke, A. D. The Journal of Chemical Physics 1993, 98, 5648. (77) Lee, C.; Yang, W.; Parr, R. G. Physical Review B 1988, 37, 785. (78) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem Phys Lett 1989, 157, 200.


Page 30 of 32

Page 31 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(79) Rassolov, V. A.; Ratner, M. A.; Pople, J. A.; Redfern, P. C.; Curtiss, L. A. Journal of Computational Chemistry 2001, 22, 976. (80) Rassolov, V. A.; Pople, J. A.; Ratner, M. A.; Windus, T. L. The Journal of Chemical Physics 1998, 109, 1223. (81) Tomasi, J.; Mennucci, B.; Cammi, R. Chemical Reviews-Columbus 2005, 105, 2999. (82) Scalmani, G.; Frisch, M. J.; Mennucci, B.; Tomasi, J.; Cammi, R.; Barone, V. The Journal of Chemical Physics 2006, 124, 094107. (83) Improta, R.; Scalmani, G.; Frisch, M. J.; Barone, V. The Journal of Chemical Physics 2007, 127, 074504. (84) Improta, R.; Barone, V.; Scalmani, G.; Frisch, M. J. The Journal of Chemical Physics 2006, 125, 054103. (85) Ogliaro, F.; Bearpark, M.; Heyd, J.; Brothers, E.; Kudin, K.; Staroverov, V.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. Wallingford, CT 2009. (86) Docampo, P.; Guldin, S.; Stefik, M.; Tiwana, P.; Orilall, M. C.; Hüttner, S.; Sai, H.; Wiesner, U.; Steiner, U.; Snaith, H. J. Advanced Functional Materials 2010, 20, 1787. (87) Bisquert, J.; Zaban, A.; Greenshtein, M.; Mora-Seró, I. Journal of the American Chemical Society 2004, 126, 13550.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents


Page 32 of 32

Protic Ionic Liquids as p-Dopant for Organic Hole Transporting

Recommend Documents